1. Field of the Invention
The present invention relates to synthetic aperture radars and, more particularly, to a synthetic aperture radar which synthesizes a monopulse antenna beam pattern.
2. Description of the Prior Art
In a conventional monopulse radar system employing a physical monopulse antenna to detect the angular deviation of a target from a boresight axis, the boresight is the electrical axis of the antenna or the angular location of a signal source within the antenna beam at which the angle error detector outputs go through zero. In a conventional monopulse antenna, several antenna beams are simultaneously produced from a single pulse and a characteristic of these beams is compared to eliminate the effect of time variance of the echo signal. Amplitude monopulse radar systems are well known in the art in which the beam produced by a first aperture of the monopulse antenna is boresighted at a small angle in relation to a beam simultaneously produced through a second aperture of the monopulse antenna. The target is detected in relation to an amplitude difference between the returns of the two beams. Phase monopulse radar systems are also well known in the art in which the beam produced by a first aperture is boresighted in parallel relation to a beam simultaneously produced by a second aperture. In this case, the target is detected in relation to a phase difference between the returns of the parallel beams. By the use of additional apertures, both amplitude and phase monopulse systems can be adapted to detect errors in both azimuth and elevation. Also, both can be implemented with reflectors fed from a point source, a line source, or a source of some other configuration.
As described above, the conventional monopulse radar systems sense target displacement by comparing the amplitude or phase of the echo signal received in each of the antenna elements, such as microwave horns. This comparison includes the use of microwave hybrids to subtract the signal returns received by pairs of antenna apertures to provide a signal output when the target is off the boresight axis. The RF circuitry for a conventional monopulse antenna subtracts the return at an element to the left of the boresight axis from the return at an element to the right of the boresight axis to sense deviation from the boresight axis in the azimuth direction. The typical RF circuitry for a two dimensional monopulse antenna also subtracts the return at an element above the boresight axis from the return at an element below the boresight axis to sense deviation from the boresight axis in the elevation direction. These microwave hybrid outputs are referred to as difference signals, which are zero when the target is on the boresight axis and which increase in magnitude with increasing displacement of the target from the boresight axis. Also in accordance with the above description, difference signal reflected from point targets on opposite sides of the boresight axis will be of a different sense. The comparison of the echo signal received in each of the antenna elements further includes the use of microwave hybrids to add the signal returns of the antenna elements to provide a maximum signal output when the target is on the boresight axis. The RF circuitry for a conventional monopulse antenna adds the return at an element to the left of the boresight axis to the return at an element to the right of the boresight axis. The typical RF circuitry for a two dimensional monopulse antenna also adds the returns at elements above and below the boresight axis. These microwave hybrid outputs which are referred to as sum signals, decrease in magnitude with increasing displacement from the boresight axis. The sum signal provides a reference signal to allow angle error detection circuitry to use the difference signal to determine the angular deviation of the targets from the boresight axis.
In conventional monopulse systems, various beam sharpening techniques have been used for many years to obtain improvement over the angular resolution limitations inherent in the beamwidth available with real aperture antennas. Various types of beam sharpening are possible using the sum and difference signals. In one technique, known as monopulse resolution improvement (hereafter sometimes referred to as MRI), the difference channel signal is adjusted in gain and is non-coherently subtracted from the sum channel signal to provide various degrees of sharpening depending upon the gain ratio, null depth, signal-to-noise ratio and other IF signal characteristics. Another technique, described here as azimuth stabilization, exploits the well known off-boresight monopulse error signal obtained by phase detecting the difference signal with the sum signal. By algebraically adding this error signal at the proper scale factor to the deflection voltage of the radar display, a vernier angle correction signal is available to correctly concentrate the signal returns from the point target at the correct spot on the display, thereby sharpening the beamwidth by improving the definition of the displayed returns.
As used with a conventional monopulse antenna, however, such beam sharpening schemes require a complex monopulse antenna with sum and difference channels and, in most cases, phase and amplitude balanced receiver channels. Against single point targets with high signal-to-noise ratios, beam sharpening factors on the order of thirty to one or more are possible over that of the basic sum channel beamwidth. Resolution improvement, however, is limited to approximately two-to-one for two or more point targets in the same resolution cell of the monopulse antenna pattern.
As a consequence of the resolution limitations experienced with conventional monopulse antennas, there has been a continuing effort to develop radar systems which are suitable for high resolution applications such as ground-mapping and air reconnaissance. Subsequent to the development of pulse-compression techniques which provided significant improvement in the range resolution of conventional radar systems, synthetic aperture radar techniques were developed for improving the angular resolution of a radar system to a significantly finer value than the angular resolution directly achievable with a radiated beamwidth from a conventional antenna of comparable dimensions. Therefore, the prior art, synthetic aperture radar techniques have been used in applications requiring substantial sharpening ratios of the real beamwidth.
The operation of a synthetic aperture radar is somewhat analogous to the operation of linear array antennas which are old in the art. A linear array antenna is a physically large antenna that is comprised of a long array of antenna elements, each of which have a relatively small aperture. The radiating elements are positioned at sampling points along a straight line and transmission signals simultaneously fed to each element of the array. The elements interconnected such that simultaneously received return signals are vectorially added to exploit the interference between the signals received by the various elements. The linear array antenna provides an effective illumination pattern that is equivalent to the illumination pattern of a single element multiplied by an array factor. The product of the single element illumination pattern and the array factor results in an effective antenna pattern having significantly sharper lobes than the antenna pattern of any single element.
Synthetic aperture radar systems are based upon the synthesis of an effectively long antenna array by signal processing means rather than by the use of a physically long antenna array. With a synthetic aperture radar, it is possible to generate a synthetic array many times longer than any physically large antenna that could be conveniently transported such that the resultant antenna beamwidth is many times narrower than the beamwidth which is attainable for a conventional radar having an antenna of comparable dimensions. Due to their synthesis of an antenna array which is much larger than the actual aperture of the physical antenna, radars using this technique have been characterized as synthetic aperture radars. In the most common synthetic aperture case, a single radiating element is used which is translated to take up sequential sampling positions along a straight line. At each of these sampling positions, a signal is transmitted and the amplitude and the phase of the radar signals received in response to that transmission are stored. After the radiating element has traversed a distance substantially equivalent to the length of the synthetic array, the signals in storage are somewhat analogous to the signals that would have been received by the elements of the conventional linear array. Accordingly, the signals in storage are subjected to substantially the same integration operation as used in forming the antenna pattern of a conventional linear array. That is, they are added vectorially. As a consequence of the non-coherent transmission from the elements of the synthetic aperture, the resolution of a synthetic aperture radar is substantially the same as could be achieved with the use of an antenna array which was twice as long as the synthetic array length. As is well known to those skilled in the art, the highest resolution achievable for a synthetic aperture radar is one-half the aperture of the physical antenna with a fundamental limit placed on the achievable resolution as a consequence of the requirement that the real beamwidth continuously illuminate the area within the synthetic array length, in combination with the inverse relationship of the real beamwidth and the aperture of the physical antenna.
Although significant improvements in angular resolution have been made through the use of synthetic aperture radars, it was also recognized that a synthetic aperture radar with further improved angular resolution capabilities would also be useful. A minimum synthetic aperture radar is capable of generating a map imagery at the average information rate of the number of resolution cells across the range extent of the map times the rate that resolution cells are encountered across the azimuth dimension of the map. However, the imagery is somewhat equivalent to a non-coherent sidelook radar which receives only a single return pulse each time the aircraft moves one beamwidth. This type of imagery is referred to as single look and has a rather coarse or grainy texture when viewed closely. Also, this imagery is subject to scintillation effects of specular echo returns which tend to obfuscate targets in close proximity. The image quality can be improved by increasing the processing rate so that additional looks are processed and combined after detection. However, the additional looks must be statistically independent, which, in the prior art, required additional processing memory as well as an increased processing rate. Generally, prior art synthetic aperture radar (SAR) systems having multilook capability have increased the amount of hardware roughly in proportion to the number of looks unless mapping performance was sacrificed since resolution can always be traded for more looks. There was, therefore, also a need for an SAR system having multilook capability which could preserve the single look resolution of the system during operation in a multilook mode without requiring substantially more hardware. That is, there was a need for an SAR system which could obtain the benefits of non-coherent integration of statistically independent looks at the target without requiring a substantial amount of additional hardware and without sacrificing the achievable resolution.
The prior art SAR systems having a high degree of resolution required a significant coherent integration time in the correlation of the stored signals. It was known that if the integration time could be reduced, certain of the motion compensation requirements of the SAR system could be relaxed resulting in a less demanding mechanization of the motion compensation apparatus for a given degree of radar platform acceleration. There was also, therefore, a need for reducing the integration time of the SAR correlator while maintaining the resolution of the SAR system to relax the requirements on the motion compensation apparatus.